The manufacture of integrated circuit devices demands precise optimal techniques for growing silicon oxide films on a circuit (wafer) substrate. This invention is intended to provide improved techniques for growing a silicon oxide film on such substrates.
Making silicon dioxide films has played a major role in the fabrication of silicon [micro-electronic] devices and in their operation since approximately 1958. When a wafer of silicon is heated in an oxidizing atmosphere, a tough hard, durable film of silicon dioxide is readily formed thereon (e.g., softening above 1400.degree. C., approximately and firmly adherent on the silicon substrate). A SiO.sub.2 layer makes an excellent electronic insulator and is very convenient to use in IC fabrication--e.g., often serving as a mask for the selective introduction of dopants.
In the art of designing and manufacturing monolithic integrated circuits, techniques for growing oxide films are extremely important; forming a subsidiary art which is highly developed and yet is still presenting manufacturing difficulties.
Now, convenient thicknesses of silicon dioxide can easily be grown in, and on, a hot silicon substrate (e.g., in oxidizing atmosphere at temperatures on the order of 1000.degree. to 1200.degree. C.)--thickness being rather precisely controlled by selecting the appropriate time and temperature of oxidation. For example, about 0.1 um. oxide will grow on substrate exposed to an atmosphere of pure oxygen for about one hour at a temperature of 1050.degree. C.;--substituting a steam medium will grow a layer about five times as thick. As workers well know, such "pyrogenic oxidation" is popular for growing precision SiO.sub.2 films because of its convenience and low cost (several hundred wafers can be simultaneously oxidized in a single run).
Now, a classical method of growing oxide on a silicon wafer substrate involves placing the wafer in a high temperature furnace and subjecting it to a flow of oxidant gas--long enough to develop the desired thickness.
Originally, such oxidation was problematic because it commonly led to an uneven oxidation rate--this causing unacceptable anomalies in the thickness and composition of the oxide film. For instance, when silicon wafers were inserted into an oven and warmed from room temp. to about 900.degree.-1100.degree. C., the initial flow of oxidant past the wafers during warm-up, produced an oxide film that was non-uniform in thickness--whereas once thermal equilibrium occurs, the oxidation rate leveled-off.
Now, workers resort to a "pre-oxidation" (or warm-up) flow to avoid this: i.e., during "warm-up" the wafers are exposed to an inert atmosphere of nitrogen (principally), with an N.sub.2 stream directed past the wafers (in a furnace) until thermal equilibrium is reached--whereupon oxidation proper begins (e.g., via a pure oxygen flow). Moreover, after oxidation was complete (end of the oxidizing cycle), a "post-oxidation" (annealing) flow of N.sub.2 was used to anneal the oxide film.
However, despite all the care that is typically taken during such thin-oxide film growing, workers know that "shorting defects" (unacceptable drop in electrical resistivity across oxide) are very prevalent--for instance, commonly leading to the rejection of about 50% or more of all the circuits on a processed wafer because of "shorts" (assuming an oxide which is "ultra-thin", i.e., less than about 500 A.degree. thick; such as a gate oxide film for an MOS circuit device). This invention teaches an improved technique for growing "thin-oxide"--a technique adapted to eliminate, or at least reduce, such shorting defects and result in improved IC yield.
I have discovered that some (if not all) of such shorting defects are due to formations of "defect particles", i.e., to the evolution of low-resistivity "micro-defects" in the oxide cross-section, these acting, in effect, as "low resistance shunts" through part, or all, of the oxide thickness. Such micro-defects will, of course, degrade the breakdown voltage (V.sub.BD) of an oxide film, especially at reduced film thicknesses--and can cause "shorting defects" of the type mentioned.
--is one "defect-limited" below 500 A.degree.?
Workers in the art of designing, making and using IC chips have for some time been painfully aware of these shorting defects (i.e., defects which reduced V.sub.BD sufficient to compromise "electrical integrity"). They have attributed them to "surface irregularities"--i.e., some sort of physical irregularity in the (Si) wafer substrate surface--and have commonly believed that this could be cured only by wafer surface-cleanup of some "super-efficient" kind and as yet unknown. Indeed, workers have for some time opined that:
"One could not render a satisfactory oxide layer thinner than about 500 A.degree. without somehow eliminating such irregularities on the substrate surface--else low-breakdown voltage would occur. That is, below 500 A.degree., one was `defect-limited`."
Quite surprisingly--and accidentally--I have discovered that this is not so! I have even found that an "electrical-integral" oxide film on the order of 300 A.degree. or less can be grown without resort to any special treatment of the substrate surface. Equally unexpectedly, I found that one parameter controlling such results was nitrogen-concentration in the pre-oxidation (and possibly the post-oxidation) sequences.
--No, but N.sub.2 concentration is critical
That is, I found that the nitrogen atmosphere of these treatments was not as "inert" as supposed (e.g., in the presence of high substrate temperatures, on the order of about 950.degree. C.), but that is appeared to induce tiny micro-defects on the Si surface such as to compromise the "electrical-integrity" of ultra-thin films below about 500 A.degree.. Further, I found that completely replacing the N.sub.2 atmosphere in such sequences--with another, "more-inert", gas such as Argon, which caused no such "micro-defects"--maintained satisfactorily-high V.sub.BD and "electrical integrity" for oxides only a few hundred A.degree. thick!
While one cannot be certain, it appears that such "bridging micro-defects" are comprised of nitrogen compounds intruding into the oxide cross section, being formed before, and/or after, the oxide growth cycle. Nitrogen gas, as such, could well induce such "micro-defect" formation (nitridation of Si wafer surface)--even where, to do so, it must penetrate a thin overlying silicon oxide layer.
Now, this was surprising (e.g., in light of what workers had been predicting--as above) and hardly predictable--indeed, as workers well know much of the chemistry of silicon is still relatively unpredictable (cf. though Si belongs to Group IV of the Periodic Table, along with Carbon, Tin and Lead, its chemistry is quite different in many respects). Workers will be surprised to learn that such nitrogen treatments can regularly generate such micro-defects on hot silicon.
Workers will attest that these discoveries are quite significant--making it so easy and convenient to form ultra-thin (&lt;500 A.degree.) oxide films on Si. For example, workers will recognize that it can facilitate the fabrication of higher-density MOS memory circuits. That is, where present-day MOS 16K-35K memory circuits involve gate oxides on the order of about 750 A.degree. in thickness, progressing to the highly-prized 64K circuits will entail dropping gate thickness to about 200-300 A.degree..
--Alternate "inert flow gases":
Now, workers might conceive that--given the evident shortcomings of Nitrogen (as a pre-oxidation stream through the furnace), any like "inert gas" might be substituted. However, only Argon was found optimally preferable in not inducing "micro-defects" (like those with N.sub.2) and also being practical and most feasible. Moreover, Helium is already in common use for "leak-checking" the furnaces of the type here prescribed (by mass spectrometry) and thus ineligible for such use.
An Xenon presents another problem: it will react with Fluorine constituents typically found on, or near, the IC wafers. The Xenon-Fluorine compounds resulting are problematic since they can form "bridging" micro-defects like those from the Nitrogen compounds, on a hot silicon substrate.
--Objects, Features:
Accordingly, it is an object of this invention to provide improved techniques for developing thin precision oxide films.
Another object of this invention is to provide improved techniques for growing silicon oxide on a hot silicon substrate and to provide improved integrated circuits so coated.
Yet another object is to teach a method of identifying "bridging micro-defects" as a source of trans-oxide "shorts" and breakdown-voltage impairment. A further object is to teach a method of identifying the cause of such micro-defects. Yet a further object is to teach a technique for substantially eliminating such micro-defects by the use of Argon as the processing atmosphere before and after forming such oxide.
As a feature of invention, I have discovered that using Argon as the furnace-medium (atmosphere) during non-oxide growing phases of the treatment sequence (e.g., before and/or after O.sub.2 flow sequence) can so eliminate such defects.
Workers in the art have, of course, used Argon for certain other purposes in various other treatments of semi-conductor substrates, such as in "Argon-implant" operations (e.g., to enhance minority-carrier lifetime). Also, it has been known to use Argon as a substituent of an annealing medium (e.g., to overcome "stacking faults"; or with an oxidizing medium (e.g., in forming oxide film on an IC substrate--and cf. U.S. Pat. Nos. 3,903,325, 3,243,314)).
A related object is to provide such improved techniques alleviating the problem of "oxide shorts" that appear when such oxide films are developed via treatments involving a nitrogenous atmosphere. A further related object is to provide such techniques for such films as applied in the production of integrated circuit devices on silicon-rich wafers whereby degradation of breakdown voltage across the film is avoided.
These and other objects and features of advantage will be seen as implemented by an improved overall oxide-growing treatment involving pre-oxidation and/or post oxidation sequences wherein an Argon-containing atmosphere contacts the substrate--especially where the oxide-growing step is preceded by a warm-up cycle and/or is followed by an anneal cycle wherein the contact medium includes Argon and no Nitrogen.
The foregoing and other features and advantages of the invention will become more apparent to workers as they become familiar with the following detailed description of the presently preferred embodiments.